The present invention relates to a heat pipe for use in waste heat recovering apparatus and the like, and more particularly to an iron heat pipe which is usable for a prolonged period of time without entailing a reduction in performance.
Heat pipes made of iron are usable over a wide range of operating temperatures including low temperatures (70.degree. to 130.degree. C.) and high temperatures (200.degree. to 400.degree. C.), less costly than those made of copper and fully satisfactory also in respect of strength. Accordingly they have found wide use. The working liquid most suitable for such heat pipes is water because water is usable over a wide range of operating temperatures and has high latent heat of evaporation (540 cal/g) and limiting heat flow (109 W/cm.sup.2). However, when water is used as the working liquid for an iron container serving as a heat pipe, the water reacts with iron to evolve hydrogen gas. The hydrogen gas diffuses in the form of stoms into the wall of the iron container and is partly released from the container at a constant rate, but a major portion of the hydrogen gas remains in the condensing portion of the container therein, consequently impairing the performance of the heat pipe. In view of this problem, it has been practice to use alkaline aqueous solutions, such as NaOH aqueous solution, as working liquids and containers formed with a corrosion preventing coating of Fe.sub.3 O.sub.4 on the inner surface. However, the coating is not fully satisfactory, permitting corrosion of the container inner surface and allowing the evolution of hydrogen gas to some extent. It is therefore impossible to limit the rate of evolution of hydrogen gas to a level not higher than the rate of release of the gas from the container and to prevent hydrogen gas from remaining within the container.